Internal combustion engines may include water injection systems that inject water from a storage tank into a plurality of locations, including an intake manifold, upstream of engine cylinders, into an intake port, upstream of cylinder intake valves, or directly into engine cylinders. One example of an engine system with multiple injector locations is shown by Brehob et al. in U.S. Pat. No. 7,426,918. Injecting water into the engine intake air may increase fuel economy and engine performance, as well as decrease engine emissions. When water is injected into the engine intake or cylinders, heat is transferred from the intake air and/or engine components to the water. This heat transfer leads to evaporation, which results in cooling. Injecting water into the intake air (e.g., in the intake manifold) lowers both the intake air temperature and a temperature of combustion at the engine cylinders. By cooling the intake air charge, a knock tendency may be decreased without enriching the combustion air-fuel ratio. This may also allow for a higher compression ratio, advanced ignition timing, and decreased exhaust temperature. As a result, fuel efficiency is increased. Additionally, greater volumetric efficiency may lead to increased torque.
However, water injection benefits may be limited by the relative humidity of the aircharge. As an example, the charge cooling effect of the water injection relies on the evaporative cooling of the injected water. However, when the air is humid, a smaller fraction of the injected water is able to evaporate, resulting in less charge cooling. The charge dilution effect of the water injection, on the other hand, relies on the presence of water dispersed in warmer air, which improves during humid conditions.
The inventors herein have further recognized that coolant flow through a charge air cooler (CAC) is used to maintain the temperature of aircharge exiting the cooler. During conditions when boost is provided by the upstream compressor, aircharge entering the cooler is hot, and the coolant flow cools the aircharge before it enters the engine. In other words, the CAC operates in a cooling mode with heat flowing from the aircharge into the coolant. However, during conditions when the aircharge entering the cooler is cold, coolant flow through the charge air cooler may be used to warm the aircharge. In other words, the charge air cooler will operate in a heating mode with heat flowing from the coolant into the aircharge. Consequently, water injection benefits may be improved by injecting water upstream of a charge air cooler during dry and cold ambient conditions while injecting water downstream of the charge air cooler during humid or warm conditions. By leveraging the different directions of heat flow at the CAC, during cold and dry conditions, a pre-CAC water injection may be used to heat and humidity the air delivered to the engine. In comparison, during hot conditions, a post-CAC water injection may be used to provide charge cooling. One example engine method includes: injecting a larger proportion of water upstream of a charge air cooler while operating the cooler in a heating mode; and injecting a larger proportion of water downstream of the charge air cooler while operating the cooler in a cooling mode.
As an example, in response to an engine dilution demand received while the engine is operated with natural aspiration, a water injection amount required to address the engine dilution demand may be determined. If the ambient conditions are dry and cold at the time of the engine dilution demand (such as when a vehicle is driving through desert regions), a larger portion of the determined water injection amount may be injected into the engine, upstream of a CAC, and a remaining, smaller portion of the determined water injection amount may be injected downstream of the CAC. In comparison, in response to an engine cooling demand (such as for knock relief) received while the engine is operated with boost, a water injection amount required to engine cooling may be determined. If the ambient conditions are warm or hot at the time of the engine cooling demand, a larger portion of the determined water injection amount may be injected into the engine, downstream of the CAC, and a remaining, smaller portion of the determined water injection amount may be directly injected into an engine cylinder or upstream of the CAC. In each case, the amount of water injected may be adjusted taking into account the condensate level at the CAC so as to reduce the likelihood of condensate-induced misfire events.
In this way, water injection benefits can be extended by using airflow through a CAC during cold, dry conditions in conjunction with water injection at a pre-CAC location to heat and humidify an aircharge before delivering it to an engine. By injecting the water at a pre-CAC location, the heat of evaporation is added to the water, increasing the overall amount of water that can be injected into the engine (relative to a post-CAC location), thereby enhancing the charge dilution effect of the water injection. By drawing heat out of coolant circulating through the CAC during cold ambient conditions, the aircharge can be heated upon passage through the CAC, without the need for a dedicated heater. By using airflow through the CAC during hot, boosted conditions in conjunction with water injection at a post-CAC location to cool the aircharge before delivering it to an engine, the charge cooling ability of the water injection is improved. By providing knock relief via a charge cooling water injection, reliance on spark retard usage is reduced, improving fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.